Why are transition-metal oxides with high oxidation numbers usually acidic?

Transition-metal oxides with high oxidation numbers are generally acidic because of the nature of their chemical bonding. As the oxidation number of a transition metal increases, the bonding in its oxides shifts from ionic to covalent.12

While transition-metal oxides with low oxidation states (+1, +2, and +3) are ionic and typically behave as basic compounds, those with higher oxidation states (+4, +5, +6, and +7) form covalent metal-oxygen bonds. Specifically, oxides with oxidation numbers of +5, +6, and +7 are acidic, meaning they react with hydroxide solutions to form salts and water. Oxides with a +4 oxidation state are generally amphoteric, meaning they can exhibit both acidic and basic properties depending on the environment.1

In summary, the increase in oxidation number correlates with a transition toward covalent character, which fundamentally alters the compound's reactivity and results in increased acidity.12